An automatic transmission that is mounted in a vehicle is set to a predetermined gear stage depending on traveling states determined based on a vehicle speed, an accelerator opening degree, or the like. This gear stage is set to allow a driving force source such as an engine to be operated in a state where energy efficiency is as high as possible and to satisfy a driving request represented by the accelerator opening degree or the like. Accordingly, in a case where an accelerator pedal is allowed to return for deceleration, for example, the engine is forced to be driven by the traveling inertial force of the vehicle and the power loss that is generated in the driving force source in that case is allowed to act as a braking force so that a driver can perform the deceleration as intended. However, the deceleration may not be the purpose of the operation for the return of the accelerator pedal in some cases and the return of the accelerator pedal may be performed so as to maintain the vehicle speed, instead of decelerating the vehicle, in some cases. In such cases, the power loss that is generated in a power source may cause fuel economy to be deteriorated.
In order to eliminate the factor that causes the deterioration in fuel economy, control for putting the transmission into the neutral state during so-called coasting, when an accelerating operation or a driving force-increasing operation is not performed, is performed. This neutral control is, in short, control for blocking power transmission between the power source such as the engine and a drive wheel. In the vehicle in which the automatic transmission is mounted, the neutral state is set by an engagement mechanism such as a clutch and a brake for gear stage setting being in a released state in some cases. When the accelerator pedal is depressed in the so-called coasting state based on the setting of the neutral state, for example, the released engagement mechanism is engaged for a return from the neutral state and the gear stage determined based on the accelerator opening degree, the vehicle speed, or the like is set.
This gear stage is to generate a required driving force, and thus it is preferable that the shift thereof is achieved quickly. A device that is used to meet the demand is disclosed in Japanese Patent Application Publication No. 2011-214673 (JP 2011-214673 A). In this device, a shift stage that is on a higher-speed side than a final shift stage (shift stage with a lower gear ratio) determined based on the vehicle speed, a driving demand, or the like is determined as an intermediate target shift stage in the case of a transition from a free-running state where the transmission is set to the neutral state to a so-called actual traveling state where a shift stage is formed. Then, shift to the final shift stage is executed after the intermediate target shift stage is temporarily set and the intermediate target shift stage is achieved.
According to the shift device that is disclosed in JP 2011-214673 A, the time taken for shift increases because an input rotation speed is significantly increased during the shift from the neutral state toward the final target shift stage while the amount of change in the input rotation speed pertaining to the case of the shift toward the intermediate target shift stage decreases because the intermediate target shift stage has a gear ratio lower than that of the final target shift stage. Accordingly, the control described above allows the overall shift time to be shortened and acceleration response to be improved. Accordingly, in the device that is disclosed in JP 2011-214673 A, the intermediate target shift stage is set first and then the driving force is generated in accordance with the gear ratio of the intermediate target shift stage in the case of a return from the neutral state to a state where a predetermined shift stage is set based on the generation of an acceleration request. Then, shift control toward the final target shift stage is initiated after the completion of the setting of the intermediate target shift stage. As described above, the shift toward the final target shift stage is initiated after the completion of the setting of the intermediate target shift stage. As a result, the time required for the setting of the final target shift stage conforming to the driver's driving request corresponds to the sum of the time taken from the neutral state to the setting of the intermediate target shift stage and the time taken from the intermediate target shift stage to the completion of the shift toward the final target shift stage. Accordingly, much time is taken until the setting of the shift stage where an intended driving force is generated after the accelerating operation such as the depression of the accelerator pedal is performed, and thus there is room for improvement regarding the improvement of the acceleration response.